... if I peeled the flap of a code from inside the issue? Really curious about this since I do this with practically all my Marvel issues (minus variants).
It's uncharted territory that's for sure. Personally, I would think it doesn't matter as those codes are only good for so long. Right now, I think it's just up to the collector whether it hurts the value, definitely not the grade though. Of course, other than variants, incentives, and the occasional 1st appearance/issue 99.9% of what you're reading isn't even worth cover price.
I'd say yes it does. The comic comes with the code so removing it is removing part of the product. Code might be useless in a few months/years but it still is a part of the issue when released. Like smkspy said, whatever issue you are 'epically destroying the value of' probably won't be worth anything for decades, if then, so you might as well get whatever you want out of the code now. Unless some character is first introduced (who goes on to be super popular) or some major event happens, almost all comics released nowadays are not going to be worth much ever.
It's def damage. since grading is insane it probably takes it from like a 9 to a 7 or something. I'm pretty sure breathing on a comic or handling it without gloves or not encasing it in lucite the minute it rolls off the presses changes it from a 10 to 9.
Not sure what it's like in the USA, but in the UK, there is practically no re-sale value in a comic. With the digital age fully upon us, nobody want's paper comics any more outside of the dying breed that the comic collector is.